Cairns to Sydney driving for 17days is probably at bit far unless you are use to driving long distances. The highway between Cairns and Sydney is mostly single lane each way with overtaking lanes every 5 kilometres, so if you are a motor home this is the opportunity for people to pass you.
Probably a 3 stage trip is best.
Cairns see Kuranda, ferry trip to the reef, the reef is closest out from Cairns although still a 1 hour boat trip out and 1 hour back.
Daintree north of Cairns about 2hours is quite good the bit I saw anyway.
You could drive to Prosperpine, which is where to take off point for islands of the Whitsundays. 7 hours driving and about 500miles. The city of Townsville is half way which is more of a working city.
www.sunlover-holidays.com.au holidays in Queensland,
www.fantasea.com.au, reef day trip company,
Most cruises I know of a 1day duration. You could stay on one of the islands for a few days.
Skip-Mackay-Rockhampton, Hervey Bay and fly straight to Brisbane.
Brisbane city itself doesn't really have that many attractions, The sunshine coast 1 hour north and Gold Coast one hour south are where the tourists hang out. Gold Coast would be more upmarket with High Rises, where the Sunshine Coast is about 15years behind in development but still very good.
Driving to Sydney from Brisbane is 14 hours, about 600miles. You could stop half way, you have to choose one, Pacific Highway via the coast passing through Gold Coast, Coffs Habour, Port Macquarie, or the Inland, New England Highway, via Warwick, Armidale and Tamworth.
Sydey don't live their but look at a harbour cruise, Sydney Opera House, Darling Harbours, The rocks.
www.exploreaustralia.com.au its a guide for travelling by car, take the book with me every I go on long distances.
www.sydney.com.au, for sites on Sydney.
All the other information people have given you seem pretty good, you just have to decide what you want to do. The places you want to cover seem about right just have to decide how to do it, what to include and what to leave out.
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Indecision is the key to flexibility